Due to their light weight, high strength and thermostability, titanium annd its alloys (hereinafter titanium) are useful metals for such aerospace applications as air frames and engine parts. Particularly effective alloys for such applications are the alpha and beta phase Ti-Al alloys as well as the high temperature Ti-Al intermetallics such as Ti.sub.3 Al (alpha 2 phase), TiAl (gamma phase) and combinations thereof which are often mixed with the alpha and/or beta alloy phases. Engine parts are commonly machined to thin cross-sections and desirably have very smooth surface finishes. Chemical milling processes are often used for this purpose whereby desired shapes, dimensions and surfaces are achieved through selective or overall removal of large amounts of metal by controlled chemical dissolution. Areas of a part where metal removal is not desired may be protected from dissolution by masking with photoresist-type masks, or the like, so as to achieve selective removal of metal from some areas of the part and not others. In the chemical milling process, the part is typically immersed in a milling solution which is agitated or flowed across the part so as to continuously present a layer of relatively fresh solution to the surface being milled and achieve a uniform metal removal rate thereacross.
Popular milling solutions for titanium contain hydrofluoric acid in concentrations varying from about 1% to about 10% often in combination with one or more other acids, such as HCl, HNO.sub.3 and H.sub.2 SO.sub.4. Other ingredients such as phosphates, dichromates, chromates, permanganates inter alia may also be present. Many of the commonly used milling solutions tend to generate considerable hydrogen at the titanium surface which results not only in relatively slow metal removal rates and rough surface finishes, but also results in an untoward absorption of hydrogen by the titanium and consequent hydrogen embrittlement of the part. Such hydrogen embrittlement seriously weakens titanium and is unacceptable in thin-section engine parts which need all the strength the metal can provide. Moreover, many of the more commonly used chemical milling solutions chemically attack photoresist the masks commonly used to protect selected regions of the part from dissolution. Accordingly the usefulness of such milling solutions for selective milling is severely limited.
It is an object of the present invention to provide a process for chemically milling titanium utilizing a milling solution which: (1) substantially eliminates hydrogen evolution at the metal surface; (2) has a relatively high metal removal rate; (3) produces smooth bright finishes; and (4) is compatible with photoresist-type masks commonly used in the selective chemical milling of titanium. This and other objects and advantages of the present invention will be more readily apparent from the detailed description thereof which follows.